Mass flow controllers (MFCs) and electronic regulators are important components of delivering process gasses (e.g., N2, O2, SF6, C4F8 . . . etc.) for semiconductor fabrication. Of particular interest are the atomic layer deposition (ALD) and three-dimensional integrated circuit (3DIC) processes which require the rapid and repeated changing or the gas species in the process chamber thousands of times to achieve the needed feature.
Changing the gas species in the chamber requires the interruption of the flow on one gas species and beginning the flow of a second gas species. One alternately turns on a Gas A and off a Gas B, and then turns off Gas B and turns on Gas A again. MFCs are normally used to turn on, turn off, and control process gas flows, however commercially available MFCs are slow to turn on and achieve controlled flow, typically having response times between 0.3 and 1.0 seconds, thereby creating a bottleneck in semiconductor processing, particularly for ADL and 3DIC processing.
Conventional techniques mitigate the processing bottleneck by using an MFC operating at a steady state and flowing into an on-off valve that opens and closes more rapidly (e.g., every 10 to 50 msec). With this approach, pressure builds up behind the on-off valve when closed during an off cycle because of the MFC continuously flows into an accumulation volume between the MFC and the on-off valve. Unfortunately, as shown in FIG. 1, when the on-off valve is opened at the beginning of an on cycle, the built up pressure in the accumulation volume initially causes a large flow of gas that quickly decays in magnitude to the steady state flow of the MFC as the stored pressure and mass is released, due to a small time constant from a low flow resistance (or nearly no flow resistance) in the on-off valve.
FIG. 1 shows a graph 100 of test data collected using special instrumentation to show an output wave produced by a system using an embodiment of the current method. During normal processing of semiconductors, instrumentation to observe the output wave in not available and thus actual flow profiles are unseen and often unknown. Problematically, a large, initial spike 120 is produced at the beginning of an on cycle. Due to the pressure build up when the on-off valve is closed, and high conductance of the on-off valve when opened, the process gas rushes through quickly in a ramp up 110 before peaking and then settling to a steady-state flow level 130 as desired. The magnitude ramps down 140 when the on-off valve is again closed during the off cycle.
The initial spike 120, however, is undesirable because it introduces an unseen, unintended, and uncontrolled event. This event can vary from system to system depending on the specific of the plumbing, air valves and supply pressure actuating the on-off valve (assuming the on-off valve is an air operated valve), and introduces a random element introducing variation in a process in which repeatability is desired. In addition, the presence of this large transient gas flow has been largely unknown and generally, large overshoots in gas flow are undesirable.
Therefore, what is needed is a technique in gas delivery systems to overcome the shortcomings of the prior art by repeatable outputting fast square waves of flow, which is reproducible from system to system, while minimizing an initial spike.